So what can those honest users do when someone buys Kindle 3G and does use for all the browsing Kindle allows but doesn't buy any single book from the Kindle store?

From Amazon's view it is not much different from tethering. In fact from data usage point it could be even worse because it is easy to ban a SIM card that clearly has traffic that normally is not possible on Kindle but not so easy when it is a regular Kindle use.